1. Field of the Invention
This invention relates to spandex having a combination of high resistance to environmental discoloration and high heat-set efficiency and, more particularly, to such spandex based on certain bulky diisocyanates.
2. Background Art
Spandex is susceptible to discoloration under certain environmental conditions, for example in the presence of atmospheric gases such as nitrogen dioxide. Improved resistance to the environment ("whiteness retention") under these conditions is a desirable attribute.
In the manufacture of tricot knits and women's hosiery, spandex yarns are often knit into the fabric with other yarns. After knitting, the fabric is generally subjected to heat-setting in order to remove wrinkles and stabilize the dimensions of the fabric. Improved heat-set efficiency is desirable to save energy, improve productivity and permit setting of the spandex in fabrics containing fibers such as cotton which cannot tolerate the high temperatures that would be necessary if the heat-set efficiency were lower.
A spandex having a combination of good environmental resistance and good heat-set efficiency is desirable, especially if the mechanical properties of the spandex were not adversely affected.
British Patent 1,102,819 discloses the use of p,p'-isopropylidenediphenyl diisocyanate, alone or in combination with a maximum of 10 mole % of other diisocyanates, to make spandex by reacting it with poly(tetramethyleneether) glycol, chain extending the resulting capped glycol with a diamine, and spinning the resulting polymer. However, the use of p,p'-isopropylidenediphenyl diisocyanate at the levels disclosed in the British patent (.gtoreq.90 mole %) results in spandex with deficient physical properties.
In general, preparation of ureas is well known. A specific two-step process is described in U.S. Pat. No. 2,929,804, Example V, using poly(tetramethylene oxide) as the glycol and 4-methyl-m-phenylene diisocyanate and p,p'-methylenediphenylisocyanate, both unhindered diisocyanates.
The use of 1,4-bis(4-isocyanato-alpha,alpha-dimethylbenzyl)benzene to make spandex is unknown.